a) Field of the Invention
The present invention relates to techniques of supplying ultra fine particles of ceramic, metal and the like not larger than about 1 μm to a substrate to form a ultra fine particle film. Such techniques of forming a ultra fine particle film are applied to the technical fields of forming a functional ceramic thin film, a metal thin film or the like on a substrate.
b) Description of the Related Art
As one of techniques of forming a ultra fine particle film, it is known to mix ultra fine particles with transport gas and spray the particles mixed with the transport gas from a nozzle toward a substrate surface to form a ultra fine particle film.
This conventional ultra fine particle film forming method is, however, associated with some problems that the film surface is not smooth and flat and the density of the film is not uniform. Specifically, with the conventional ultra fine particle film forming method, if ultra fine particles contain defective particles (such as particles having a diameter of 1 μm or larger and insufficiently accelerated particles) unable to physically form a film, these defective particles are mixed in a deposit on a substrate.
More specifically, if ultra fine particles jetted out during a film deposition contain a particle (defective particle) 31 having a large particle diameter or an insufficient speed, as shown in a schematic diagram of FIG. 9A and a microscopic photograph shown in FIG. 10A, the large diameter defective particle 31 attaches to and sinks in the surface layer of a deposit 32 under growth, and this defective particle 31 functions as a mask so that deposition does not occur thereafter on the surface of this defective particle 31. As shown in FIGS. 9A and 10B, the film 33 after cleaning has a depression 34. As shown in a microscopic photograph of FIG. 11, the film surface is very rough, which adversely affects later deposition. Since the defective particle is in a floating state in the deposit 32, the film is not dense and the surface of the deposit is abraded by ultra fine particles to be later blown toward the surface. Also in this case, as shown in FIG. 9B, the film 33 has a depression 34 and the film surface is very rough, which adversely affects later deposition. If such a conventional film forming method is applied to forming a ceramic electronic component which is required to have a homogeneously controlled fine structure of the film, excellent electrical characteristics cannot be expected.
It is also practically difficult to make the amount of fine particles to be jetted out of a nozzle uniform and constant, so that the film thickness changes at positions. It is also difficult to control a film thickness and a surface uniformity, which becomes a large obstacle against forming films having uniform performance and good film quality. Such irregular film thickness and surface roughness become a critical issue when the conventional method is applied to forming an optical thin film.
It has been desired to provide techniques of forming a ultra fine particle film which has ultra fine particles sufficiently bonded together, sufficient density, flat surface and uniform density.